1. Field of the Invention
The present invention relates to an improved enhancement mode field effect transistor (FET).
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Enhancement mode (E-mode), or normally-off devices, based on Gallium Nitride (GaN) technology are interesting for a variety of applications, for example, in integration of control circuitry and for the added safety of a normally-off device in power switching applications. Enhancement mode operation is commonly achieved using an AlGaN/GaN buffer structure, by etching away some of the AlGaN under the gate region until all the charge is depleted [1], or by exposing the AlGaN under the gate with fluoride-based plasma until negatively charged fixed fluorine ions screen all the charge in the channel [2]. These devices suffer from threshold voltage non-uniformity and repeatability, due to the processes requiring gate recess etch or plasma treatment. Also, these devices have a low Schottky gate turn-on voltage (of at most 2 V) due to low Schottky barriers. If a threshold voltage of 1 V is required, these devices are left with a maximum modulation of 1 V. Because high-power switching applications require a threshold voltage of over 1 V for gate signal noise immunity, increasing the gate turn-on voltage is crucial.
Utilization of p-GaN barrier below the gate [3,4] depletes the channel and increases the gate turn-on voltage to 3 V, rendering it attractive for high-power applications. However, such field effect devices suffer from high on-resistance. The present invention seeks to reduce the on-resistance.